The present invention relates to an improved moisture-permeable sheet, and to a process for the preparation of the same.
More specifically, it pertains to a sheet consisting of a thin-layer film having a thickness of the order of 0.2 mm and possessing elasticity, mechanical strength and water permeability, and to a process for the preparation of a sheet having moisture permeability which permits an optional control of water permeability. The sheet obtained in accordance with the present invention is primarily applicable as a wig base to be planted with natural or artificial hair. The present sheet may also be used as artificial skin and a general-purpose sheet.
It has now been found that currently available wig bases comprising mainly a urethane elastomer have less moisture permeability and air permeability. In general, the amount of sweat emanating from man's skin is 350 g/m.sup.2 /24 hr on the average, depending upon the condition of health. Accordingly, the prior art wig base becomes stuffy on wearing, and does not stand up to use over an extended period due to its accelerated deterioration.
Scores of patent specifications have been filed, which disclose processes for improving the air or moisture permeability of polyurethane elastomer sheets (see, for instance, Japanese Patent Publications Nos. 55-35503 and 54-24428). However, these specifications chiefly teach the application of sheets for making synthetic leather or non-woven fabric, and suggest nothing about the use of the sheets as air or moisture permeable wig bases having a thickness of the order of 0.2 mm.
A film-forming dope composed mainly of a poly-.alpha.-alkyl-L-glutamate, that is a polypeptide of a .alpha.-alkyl-L-glutamic acid, is sold on the market under the trademark AJICOAT by Ajinomoto Co., Inc. This material can provide a thin film with a thickness of about 30 microns and a considerable tensile strength, but exhibits no rubber elasticity as such owing to its linear polymer structure. For this reason, that material cannot be used as a wig base over a longer period of time.
Recently, the composition of (co)polymers of amino acid derivatives other than glutamic acid has been open to the public. The (co)polymers excel not only in moisture permeability but also, particularly, in oxygen permeability, and are used as artificial skin for treating a burn; however, they possess less mechanical strength and no elasticity.